Old slip made of superior substance perhaps (6,6)
I believe the answer is:
senior moment
'old slip' is the definition.
(slip or error made due to age)
'superior substance perhaps' is the wordplay.
'superior' becomes 'senior' (similar in meaning).
'substance perhaps' becomes 'moment' (being substantial or momentous).
'senior'+'moment'='SENIOR MOMENT'
'made of' is the link.
(Other definitions for senior moment that I've seen before include "Fleeting loss of memory" , "memory lapse" .)